Treatment of latex



Aug. 4, 11936 H. P. STEVENS TREATMENT OF LATEX,

Filed Dec. 12, 1954 FIGI.

' FIG-.5.

C. m m7 W w a e .V Wk a I I y 0 Patented Aug. 4, 1936 TREATMENT OF LATEX Henry Potter Stevens, London, England, assignor to Rubber Producers Research Association, London, England Application December 12, 1934, Serial No. 757,212 In Great Britain December 18, 1933 8 Claims. (o1. 1s-5o) This invention relates to the purification of improved method of concentrating latex, whererubber latex and has more especial reference to in concentration proceeds concurrently or consecprocesses for the purification and simultaneous utively with purification as one uninterrupted or subsequent concentration of rubber latex. process.

The latex of rubber-bearing trees is known to In effecting a practically useful degree of puri consists in the main of rubber or othergums, resfication an appreciable dilution of the latex ocins, proteins, and similar substances as a suscurs by the entry of water under the influence of pension in an aqueous serum carrying carbohyosmotic forces, unless these forces are opposed drates, salts and other substances which are waeither by excess hydrostatic pressure on the latex i ter soluble and therefore held in solution in the side of the diaphragm or by the utilization, inlatex. stead of water, on the side opposed to the latex,

In order to utilize this latex to the best adof appropriate aqueous solutions such as solutions vantage, it is desirable, and in some cases essenof ammonium soaps which exert an osmotic force tial, to remove part of the aqueous content, thereopposed to that of the latex, or by both these 5 by obtaining a more concentrated suspension or methods in conjunction. By such methods it has dispersion of the rubber or other colloidal conbeen found that dilution can not only be retarded it F r other p rp it is a desi l but that the flow of diluent can be reversed and or essential to remove the water soluble or crysconcentration of the latex thereby efiected. talloid constituents, and thereby to purify the With the methods of concentration hitherto latex. exploited, the water soluble impurities or a por- 20 According to the present invention, the latex is tion thereof are retained with the concentrate, p fi d by a dialyzing treatment, the carbohybut this disadvantage does nct apply to the latex (hates, Salts and Other crystelloid p es concentrated after purification in accordance with eluding hydrolyzed p ote if p e held n the present invention, since the purification can 35 when passing through t membrane, While the be continued to any desired extent and the con- Colloidal rubber particles 1 not D hydrostatic centration carried out to any desired degree, for pressure be ex e o the y of the lateX instance to yield a fluid concentrate with a dry t pr m t d a ys s y movement of the latex rubber content of 60% to 65% or a paste with a and simultaneously to retard or reverse the norrubber content of 80% to 85%. Similarly, where mal dilution which would otherwise occur during the processes are arranged to effect concentration 30 dialysis. either wholly or in part simultaneously with purl- The d ys y be e With Water on fication, the water soluble impurities are removed t e Opp Side Of t membrane through Which from the concentrate and the disadvantage hith-' diffusion of the water soluble non-caoutchouc g experienced is b iat d,

constituents of the latex takes place. By'appro- AS compared ith th particles f many 1- priate control Of the W g Conditions, the loidal substances which can be separated by dialykind and thickness o membrane. the relative sis, the rubber particles are relatively coarse so volumes of rubber latex and o a the depth that the separation and consequent purification or thickness of the latex y the temperature, proceeds with case, although, sin'ce dialysis is a the Pressure and the movement 0f the latex and relatively slow operation and the ammonia usu- 40 of water, control can be exerted on the rate and n added t t l t as preservative or t extent of purification. bilizer is a crystalloid substance which will there- The p eferr d pr ess according to the invenfore pass through the membrane with the water tion includes two stag the fi effecting D soluble impurities, it may be desirable to renew 45 fieation 0f e latex y dialysis either W the ammonia content by further additions. The

without pa t concentration and the second loss of the ammonia during dialysis does not necstage the concentration of the thus purified latex essarily lead to premature coagulation of the by any usual or appropriate treatment such as latex, however, probably because the impurities centrifugation, creaming or evaporation. which pass through the membrane and are thus The invention, however, also comprehends an separated are also those which normally cause 0 fermentation or putrefaction or promote the formation of acidic substances, any of which occurrences renders the latex unstable and prone to coagulate. It will be appreciated that as the purification according to the invention proceeds and such detrimentally active materials are removed, the liability of the latex to be unstable is lessened, but since it is very often desirable to en sure the stability of the latex during the subsequent concentration, it may be advisable to add a small quantity amounting to about .5% of ammonia which addition may,' where desired, be made during the course of the dialyzing treatment.

Reference will now be had to the accompanying drawing, which illustrates diagrammatically apparatus suitable for carrying out the process according to the invention, and in which,

Fig. 1 is a diagrammatic illustration of a water cell, and Fig. 2 a similar illustration of a latex cell, Fig. 3 showing a plurality of water and latex cells assembled alternately in series with the connections by'which latex is fed to the alternate cells and maintained therein under pressure.

Each dialyzer cell consists of a hollow square frame I, conveniently of wood, bounded on either square face'by a sheet of dialyzing membrane 2, numbers of these cells being assembled to form a working unit as shown in Fig. 3 and being held together by clamps or bolts 3.

The thickness of the cells may conveniently be from to 1", and it will be appreciated that the apparatus may be suited to the treatment of small or large quantities of latex by a simple reduction or multiplication of the dialyzer cells.

Alternate cells are charged with latex and the intermediate cells with water, and the latter contain a woven wire mat I or other form of spacer to prevent collapse on to one another of the opposed membranes or diaphragms which form the walls of the cells.

The latex cells have ho spacers as they are continuously held at a pressure greater than 'atmosphere plus the water head in the water cells, the apparatus being closely similar to one form used in the recovery of caustic soda from alkalicellulose liquor in the artificial silk trade, but

' having'the addition of means by which air pressure may be applied to the latex cells mainly to hasten dialysis and retard dilution but also to move the latex from the cells to the storage tank, and if desired to move the latex from cell to cell in order to promote the dialysis.

The apparatus is also furnished with means for feeding in and discharging the latex and water to and from their respective cells, the water being renewed as often as desired during the treatment of one batch of latex through a pipe 5 connected to a suitable source of supply.

' 6 is a tank containing the bulk of the latex to be treated and is connected through a pipe system I to the latex cells designated 8. 0n the other hand the tank 8 is connected through a pipe system 9 and suitable valves I0 and H with a source of air under pressure so that the latex cells 8 may be re-charged as requisite.

Valve I0 is also operable to admit air under pressure directly to the latex cells through a pipe system I2 to prevent dilution of the latex under treatment by water passing inversely through the membranes 2.

The following example indicates the degree of purification achieved when employing three square feet of vegetable parchment paper as dialyzing membrane per gallon of latex under lger 109 Percent y welg o excess of latex total solids (coagulable over caoutchouc) caoutchouc Original latex 37. 8 41. 2 9. 0 Purified latex 37. 2 37. 7 l. 3 Purified and then concentrated latex 56. 0 l. 4

During the dialyzing treatment forming the first stage of the process by which the above results were achieved a pressure of 1 to 2 lbs. per square inch in excess of atmospheric pressure was maintained in the latex cells, while the water in the cells was open to atmosphere. Ordinary room temperature obtained.

The dialysis may, however, be carried out at any convenient temperature within the range over which coagulation of the latex does not take place. With other conditions similar there is an increase in the amount of dialyzate obtained in the first 20 or 24 hours of about 3% per 5.0" C. rise of temperature over the range 20 to 50 C.

The preferred membrane is a vegetable parchment paper and although animal membranes, e. g.

- gold beaters skins, may be used with high efilciency, the assembly of such material in large sheets suited to the working conditions and the high cost, render their use expensive. Alternatively, closely woven cloths may be used and may be padded or treated with film forming substances such as cellulose derivatives, rubber or the like. In practice, however, parchment paper varying from 60 to grams per square meter in weight have been found very suitable. 4

For latices containing dialysis-resistant noncaoutchouc constituents, the dialysis may be preceded by treatment promoting a process of hydrolysis of such constituents, e. g. by warming with or merely storing for a time with alkali or by the addition of a suitable enzyme whereby such constituents are broken down into dialyzable bodies and thus facilitate and further the subsequent process of purification.

The following example will indicate the advantage accruing from the use of pressure on the latex to assist the process of purification, a latex dialyzed through vegetable parchment paper for 20 hours at 20 to 23 C. yielding the following results.

It will be seen that in addition to substantial concentration of the latex simultaneously with purification, the pressure on the latex has also had the effect of increasing the mass of crystalloid impurities expelled from the latex, thus enhancing the degree of purification.

Initially there may be some dilution of the latex but as the dialysis proceeds even light pressures on the latex are suflicient to cause a flow of water against the osmotic pressure and in the same direction as the crystalloid substances, the following increase having been obtained with a layer of latex 6 to 7 m.ms. deep at 50 C. and under a pressure of about 1 lbs. per square inch.

Water entering or leaviu 100 grams atex Hours from trees The final figure represents an increase in concentration from the original 37.5% to about 58.5% dry rubber content.

By ascertaining the difference between the total solids and the coagulable caoutchouc obtainable from the latex before and after purification-concentration, an indication of the degree of purification can be had, a typical latex treated by the process to increase the rubber content from 37.5 to 48.9% showing a 6.8% excess of total solids before treatment and only a Per 100 grams latex Excess of total solids (aver caout- Coagulable Total c caoutchouc solids 2 3 3 Before dialysis 36. 2 40. 6 12. 2' After dialysis 37. 7 38. l 1. 06

This proves that 91% of the non-coagulable constituents were removed.

Nitrogen content Nitrogen per Material 100 grams of material Coa ulable caoutchouc from undialyzed latex 0.33 Tots] solids from undialyzed latex 0.60 Total solids from dialyzed latex 0. 36

as tested by the percentage removal of noncoagulable nitrogen i. e. 89%, is very close to the degree of purification as measured by the removal of total non-coagulable dissolved solids, i. e. 91%. 5

In another example the nitrogen content of the dry dialyzate itself was determined and was found to'be 2.3% corresponding to the presence, in the dialyzate of 12% to 14% of hydrolytic products from proteins. 10

These are examples of the degree and nature of the purification effected on normal ammonia preserved latex, not treated before dialysis to produce any proteolytic product. But the process may be varied by such pre-treatment and by so 15 doing reduce the amount of coagulable non-caoutchouc constituents, with corresponding increase in dialyzable matter and degree of purification of the caoutchouc.

A preliminary concentration of the latex may 20 be eilected before the purification and concentration according to the invention, such preliminary concentration having the effect 01' augmenting the relative crystalloid content and being adopted, with advantage, where it is desired 25 to use a stabilizer, e. g. excess of fixed alkali during concentration, and to remove same during the dialyzing treatment.

Protective colloids may supplement or replace the ammonia addition to assure the resistance of 30 the latex against coagulation where it is concentrated by a subsequent treatment, the choice and quantity of the stabilizer being dependent on the particular process of concentration adopted.

What I claim is:-

l. A process for the purification of rubber latex consisting in removing by dialysis with water or with an aqueous solution carbohydrates, salts or other crystalloid impurities held in solution and in exerting hydrostatic pressure on the body of the latex to promote dialysis by movement of the latex and simultaneously to retard or reverse the dilution normally'occurring during dialysis.

2. A process for the purification of rubber latex consisting in removing carbohydrates, salts and other crystalloid impurities held in solution by dialysis with an aqueous solution exerting an osmotic force opposed to that of the latex and thereby retarding dilution of the latex during the treatment.

3. A process for the purification of rubber latex according to claim 2, wherein the latex is dialyzed with an ammonium soap solution.

4. A process for the purification of rubber latex consisting in removing carbohydrates, salts and other crystalloid impurities held in solution by dialysis with water or with an aqueous solution, the temperature being maintained in the neighbourhood of 50 throughout the treatment thereby to accelerate dialysis.

5. A continuous process for the purification and simultaneous concentration of rubber latex according to claim 1, wherein the body of the latex is supplied in parts to alternate dialyzer cells under pressure and wherein intermediate cells contain water or an aqueous solution for the receipt of dialyzate, the intermediate cells being replenishable as required.

6. A process for the treatment of rubber latex according to claim 1, consisting in preceding the dialysis by hydrolysis of the non-caoutchouc constituents, e. g. by warming the latex or storing it for a time with alkali.

7. A process for the treatment of rubber latex according to claim 1, wherein a pressure of from 1 to 2 lbs. is maintained on the latex during dialysis to counteract dilution thereof.

8. A process for the purification of rubber latex consisting in removing by dialysis carbohydrates, salts and other crystalloid impurities held in so lution, and in retarding or reversing the normal dilution of the later; during dialysis with or without hydrostatic pressure exerted on the body of the latex, by the employment on the side of the membrane opposite to the latex of an aqueous solution not capable of permeating the membrane and exerting an osmotic force opposed to that of 5 

